There has been extensive research into the development of micropumps. These research efforts include pumps based on oscillating piezoelectric membranes, peristaltic pumps, electrohydrodynamic pumps, and others. These pumps, to date, appear to be incapable of generating the pressure and/or flow in a compact and reliable device.
The phenomenon of electro-osmosis has been known since the work of F. F. Reuss in 1809. A simple description of this phenomenon is that liquid flow is induced on a region of net charge that develops at the liquid/wall interface known as the electric double layer. The charged fluid layer occurs as a result of a spontaneous chemical reaction between the surface and the electrolyte in the fluid that generates the charge separation at the liquid/solid interface called the “electric double layer”. An externally imposed electric field forces the diffuse ions (charges) in the electric double layer (EDL) into motion. The diffuse ions drag the neutral bulk liquid. The magnitude of the force is proportional to the applied electric field, and the quantity of the charged species available in this region of net charge. Larger flow rates can be achieved for systems with large cross-sectional areas. Large pressure generation requires flow channels with small characteristic length scales. Porous structures and many small flow channels in parallel provide structure with high surface to volume ratio.
Miniature pumps based on the phenomenon of electro-osmosis (i.e., electroosmotic pumps) were originally developed by Theeuwes (U.S. Pat. No. 3,923,426), in which a porous ceramic structure was used to provide a multitude of micron-sized pathways with charged surface layers. Theeuwes describes the importance of selecting pumping structures which feature high porosity, high electroosmotic mobility for a given working fluid, small diameter pores, and discusses the possibility of the use of quartz or glass ceramics, possibly comprised of beads, and porous polymer matrices. The working fluid in the Theeuwes pump was suggested to have a high dielectric constant, low viscosity, and low electrical conductivity. Example liquids that the Theeuwes pump used include deionized water, ethyl-alcohol and alcohol-water mixtures, and many organic solutions.
Moreover, there exists a need for a highly reliable miniature pump that is capable of generating the high pressure (for example, pressure greater than 10 PSI) and/or high flow (for example, a flow rate greater than 0.01 ml/min) that are necessary for various applications in the biochemical assay space. Such a pump should overcome or address the shortcomings of the conventional pumps. EO pumps are compact devices and/because they have no moving parts are potentially very reliable.